The present invention relates to the measurement of algebraic values of a magnetic field produced by an alternating current. This measurement is performed on a moving body to be positioned and which carries magnetometers.
The invention more particularly applies to the dynamic anchoring of a ship to a platform in the open sea. This platform has two conductors which are submerged in the water and through which pass in opposite directions an alternating current. The ship carries magnetometers, which measure the components of the instantaneous magnetic field in accordance with two axes Ox and Oy associated with the ship.
It is known from U.S. Pat. No. 4,316,253 to determine the relative signs of the components h.sub.x and h.sub.y of the magnetic field with respect to the moving axes Ox and Oy by a synchronous demodulation of one signal such as h.sub.x by the other signals such as h.sub.y. It is possible to position the ship on the basis of the knowledge of the algebraic values of h.sub.x and h.sub.y and the course angle of the ship, when the chart of the magnetic values is known.
The synchronous demodulation of h.sub.y by h.sub.x produces an ambiguity at the start of the position measurement because, compared with the signs of the instantaneous current producing the magnetic field, the signs of h.sub.x and h.sub.y are not known. It is known to eliminate doubt by a rough position measurement.